With respect to barriers, the change in APIs used to achieve the same task. For example Chrome, Edge, Opera, Brave support the extension APIs from Chrome. Whereas Firefox & Safari support the extension APIs from Mozilla. Also there is an ongoing push from Chrome to move to manifest v3 which brings in new features & security (service workers, extended scope), but that leads to rework for extensions developed in manifest v2. So, there is always a catchup to be done, if browsers introduce new APIs or change the way the APIs should be used then there is a constant effort required to keep the extensions working.

I have a number of popular browser extensions (10,000+ users). I don't see any barriers. It's quite easy to do, fun and profitable.

My feeling about browser extensions is that half the ideas for good ones rely on third party websites.

So if I make a popular extension, I open myself to stress (eg: frantically updating my extension to respond to the third party pushing an update).

I've seen quite a few popular extensions die due to browser changes that removed APIs. Somehow that sort of rug-pull doesn't seem to happen much with websites or mobile apps.